The Iranian calendars or Iranian chronology (Persian: گاهشماری ایرانی, Gāh-Šomāri-ye Irāni) are a succession of calendars invented or used for over two millennia in Iran, also known as Persia. One of the longest chronological records in human history, the Iranian calendar has been modified time and again during its history to suit administrative, climatic, and religious purposes. The most influential face in laying the frameworks for the calendar and its precision was the 11 century Persian polymath, hakim Omar Khayyam. The modern Iranian calendar is currently the official calendar in Iran. It begins at the midnight nearest to the instant of the vernal equinox as determined by astronomic calculations for the Iran Standard Time meridian (52.5°E or UTC+03:30). It is, therefore, an observation-based calendar, unlike the Gregorian, which is rule-based.
The Iranian year usually begins within a day of 21 March of the Gregorian calendar. A short table of year correspondences between the Persian and Gregorian calendars is provided below.